The role of muscular traction in the occurrence of skeletal relapse after advancement bilateral sagittal split osteotomy (BSSO): A systematic review

Abstract The aim of this systematic review was (i) to determine the role of muscular traction in the occurrence of skeletal relapse after advancement BSSO and (ii) to investigate the effect of advancement BSSO on the perimandibular muscles. This systematic review reports in accordance with the recommendations proposed by the Preferred Reporting Items for Systematic Reviews and Meta‐Analyses (PRISMA) statement. Electronic database searches were performed in the databases MEDLINE, Embase and Cochrane Library. Inclusion criteria were as follows: assessment of relapse after advancement BSSO; assessment of morphological and functional change of the muscles after advancement BSSO; and clinical studies on human subjects. Exclusion criteria were as follows: surgery other than advancement BSSO; studies in which muscle activity/traction was not investigated; and case reports with a sample of five cases or fewer, review articles, meta‐analyses, letters, congress abstracts or commentaries. Of the initial 1006 unique articles, 11 studies were finally included. In four studies, an intervention involving the musculature was performed with subsequent assessment of skeletal relapse. The changes in the morphological and functional properties of the muscles after BSSO were studied in seven studies. The findings of this review demonstrate that the perimandibular musculature plays a role in skeletal relapse after advancement BSSO and may serve as a target for preventive strategies to reduce this complication. However, further research is necessary to (i) develop a better understanding of the role of each muscle group, (ii) to develop new therapeutic strategies and (iii) to define criteria that allow identification of patients at risk.

The occurrence of SR has been largely attributed to increased soft tissue and muscular tension due to mandibular advancement. 18 This may also explain the higher incidence of SR in patients with a high MPA, as the muscles of mastication are stretched in the ramus area when the proximal segment is rotated in the counterclockwise direction. 4 Furthermore, this hypothesis can possibly explain the major differences in stability between rigid internal fixation (RIF) and wire fixation, as RIF might be more resistant to dorsal traction by the perimandibular muscles on the advanced mandibular segment. This muscle tension hypothesis was further investigated by Ellis and Carlson, who conducted an animal study on 10 rhesus monkeys in which the mandible was surgically advanced. 19 In five of these animals, an additional myotomy of the suprahyoidal (SH) muscles was performed. In the non-myotomy group, the mean SR was 13.19%  Considering the factors associated with the occurrence of SR, how the influence of all these factors may be explained by this muscle tension theory and the findings of the animal study by Ellis and Carlson, it can be suggested that muscular stretch may play a role in the occurrence of SR after BSSO advancement.
The primary aim of this systematic review was to determine the role of muscular traction in the occurrence of SR after the BSSO advancement. The secondary aim was to investigate the effect of BSSO advancement and the resulting dentoskeletal changes on the perimandibular muscles.

| MATERIAL S AND ME THODS
This systematic review was conducted in accordance with the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. 20 The protocol for this systematic review was registered on the International Prospective Register of Systematic Reviews (PROSPERO).

| Focused question
The review focused on the following research question: Does muscular activity/traction play a role in the occurrence of SR after advancement BSSO?
A secondary research question was as follows: Does BSSO advancement affect muscular activity/traction after surgery?

| Search strategy
To identify the relevant studies, a systematic search was carried out in the MEDLINE, Embase and Cochrane Library databases using the modified PICOS strategy displayed in Table 1

| Study selection
Study selection was performed by two independent reviewers (VDB and SV). Disagreements regarding entry were resolved by consensus.
The inclusion criteria were as follows: (i) assessment of SR after BSSO advancement; (ii) assessment of morphological and functional changes in the perimandibular muscles after BSSO advancement; and (iii) clinical studies on human subjects.
Following the removal of duplicates, all articles were screened by title. Titles that were not relevant to this review were also excluded.
Subsequently, the remaining articles were evaluated using an abstract. The following exclusion criteria were applied: (i) surgery not comprising BSSO advancement; (ii) studies in which muscle activity/ traction was not investigated; and (iii) case reports with a sample of five cases or fewer, review articles, meta-analyses, letters, congress abstracts or commentaries. The full texts of selected articles were included in this systematic review. In addition, the reference lists of the included studies were screened to identify additional publications eligible for inclusion.

| Data extraction
For the analysis of the influence of the perimandibular muscles on SR, the primary outcome variable was SR, as measured by imaging.
The primary predictor variable was perimandibular muscle action. To analyse the influence of BSSO advancement on the perimandibular muscles, the primary outcome measure was morphological changes of the perimandibular muscles. Secondary outcome measures included functional changes such as maximum and relative strength.
The primary predictor variable was the degree of mandibular advancement.
Each study was evaluated for the following variables: year of publication, study design, sample size, age and sex of the participants, pre-operative angle classification, treatment, fixation technique, type and duration of retention, assessed outcomes, method of outcome assessment, reported outcomes and findings. These data were extracted from each study by both reviewers (VDB and SV). In case of disagreement between the reviewers, a discussion was undertaken until an agreement was reached. Custom-made forms were used for the data extraction.

| Risk of bias assessment
The Risk Of Bias In Non-randomized Studies of Interventions (ROBINS-I) tool was used to assess the risk of bias in the included studies. 21 This tool covers seven domains through which bias might be introduced, that is confounding, selection of participants, classification of interventions, deviations from intended interventions, missing data, measurement of outcomes and selection of the reported results. The domains were classified as low (-), moderate (±), serious (+) or critical (++) risk of bias.
The two reviewers (VDB and SV) independently assessed the risk of bias in the included studies. Disagreements regarding entry were resolved by discussion.
When the risk of bias could not be assessed properly due to lack of information in the published paper, an attempt was made to contact the corresponding author for clarification.

| Data synthesis
The high heterogeneity of the reported outcome measures did not allow the quantitative synthesis of the data retrieved from the included studies. Therefore, a qualitative synthesis of the results was performed.

| Search results
The process of selection and inclusion of articles is illustrated in Figure 1. The initial searches of the MEDLINE, Embase and Cochrane databases yielded 591, 361 and 54 studies, respectively.
All 1006 records were checked for duplicates. After removal of duplicates, the remaining articles (n = 865) were screened by title, uncovering 75 potentially eligible articles. Of these 75 articles, 37 articles were excluded after reading the abstract against the preset exclusion criteria: surgery other than BSSO advancement (n = 29); studies in which muscle activity/traction was not investigated (n = 5); and case reports with a sample of five cases or fewer, review articles, meta-analyses, letters, congress abstracts or com- For the remaining 38 articles, the full text was retrieved and analysed in full detail. Twenty-eight articles were excluded based on the full text for the following reasons: surgery other than BSSO advancement (n = 14); studies in which muscle activity/traction was not investigated (n = 7); case reports with a sample of five cases or fewer, article reviews, meta-analyses, letters, congress abstracts or commentaries (n = 4), full text unavailable (n = 2), and results presented in other included articles (n = 1). The remaining 10 articles were included in the qualitative synthesis. [22][23][24][25][26][27][28][29][30][31] Screening of the reference lists of included studies yielded one additional study that met the inclusion criteria. 32 A total of 11 articles fulfilled the inclusion criteria and were included in the final qualitative synthesis. 22-32

| Study characteristics
The characteristics of each included study are listed in Table 2.
Eight of the 11 included studies had a prospective study design, 22 In eight studies, [22][23][24]27,[29][30][31][32] rigid internal fixation was used, whereas in two studies 25,28 osteosynthesis was performed using wires. The fixation technique was not specified in one study. 26 In three studies, 24,27,28 rigid intermaxillary fixation was used for postoperative retention, whereas in two studies 23,31 guiding elastics were applied. In three other studies, no post-operative retention was used. 29,30,32 The post-operative retention protocol was not specified in three studies. 22,25,26 Four of the 11 included studies aimed to investigate the effect of muscular activity on the occurrence of SR. 22,23,28,31 The seven other studies focused on the adaptation of muscles after BSSO. [24][25][26][27]29,30,32 Of these seven studies, four studies investigated the morphological changes of the muscles, 27,29,30,32 whereas the three remaining studies investigated the functional changes. 24-26

| Risk of bias assessment
The quality of the included studies according to the ROBINS-I tool for assessing the risk of bias in non-randomized studies of interventions is shown in Table 3. There was an overall low risk of bias due to the classification of interventions or deviations in the intended interventions. However, as the treatment (BSSO or bimaxillary surgery) was not specified for each group in all studies, and the outcome assessment was not blinded, the risk of bias due to confounding factors and the measurement of the outcomes was considered moderate. The risk of bias due to missing data was also moderate as a result of dropouts during follow-up in some studies. Due to the fact that the amount of advancement was often not reported, there could be a significant risk of bias due to the selection of the reported outcome. Finally, the most serious risk of bias was found in the selection of the participants due to either participant selection after the intervention or lack of a detailed description of the protocol for participant selection.

| Perimandibular muscles and relapse
Four studies assessed the effect of interventions on the occurrence of SR. 22,23,28,31 The characteristics of these studies are summarized in Table 4.
A total of 156 subjects with class II malocclusion were treated with BSSO advancement. 22,23,28,31 In 70 subjects, one of the follow-

| Morphological and functional changes of the perimandibular muscles
Morphological and functional changes in the perimandibular muscles after orthognathic surgery were investigated in four and three studies, respectively. [24][25][26][27]29,30,32 The characteristics and main findings of these studies are summarized in Table 5. Statistical analyses showed only a weak correlation between initial SH stretch and long-term SR. However, there was a significant correlation between muscle complex lengthening during post-operative follow-up and SR. The morphological changes of the masseter muscle and the MPM after BSSO were investigated by Dicker et al. 29,30,32 In the first study, MRI was used to assess the volume and maximal cross-sectional area of these muscles pre-and post-operatively in 12 retrognathic subjects undergoing BSSO advancement. 30 The results showed a significant decrease, up to 18%, in both volume and maximum cross-sectional area of the jaw muscles 10-48 months postoperatively. A second study was performed by the same research group to evaluate the change in muscle direction and moment arms of bite force 32 and showed a significant change in the vertical direction of both the MMs and MPMs in the sagittal plane among the subgroup of subjects with an MPA greater than 39° treated with bimaxillary surgery. In a third study, data from the same subjects were used to generate a biomechanical model to assess the muscle forces on the condyle and the corresponding joint reaction force. 29 The findings of this study showed only minor increases in the joint reaction force after surgery.
Wessberg and Epker evaluated the influence of the modified sagittal split technique, as described by Epker, 34 on masticatory muscle function using EMG and kinesiometry. 25 The results showed no significant changes in the interocclusal space or EMG activity of the masticatory muscles three months after surgery compared to the pre-operative measurements. In a second study, the excitation pattern of the MM in both groups of treated subjects, Angle class II (treated with BSSO) and class III (treated with bimaxillary surgery), was compared to eugnathic controls (untreated) before and after surgery. 24 The initial pat-

| Perimandibular muscles and relapse after BSSO advancement
The first study investigating the role of muscular traction in the occurrence of SR after advancement BSSO was published in 1982 by Wessberg et al. 28 The authors concluded that SR is not significantly altered by SH myotomy; therefore, the SH muscles cannot be primarily responsible for SR after BSSO advancement. This conclusion is supported by the finding of Van der Linden et al that additional stretch of the SH muscles, by simultaneous advancement genioplasty, was not associated with an increase in the occurrence of SR. 23 Dicker G. 2007 30 Eggensperger N. 2005 27 Eckardt L. 1997 24 Wessberg G. 1982 28 Wessberg G. 1981 25

Author, Publication
Year  The evidence provided by the studies in this review indicates that the perimandibular musculature might serve as a possible target to prevent or reduce SR after the advancement of BSSO. However, it has to be emphasized that the available evidence is limited, and further research will be necessary to validate the benefit of current interventions in different subgroups of subjects in well-designed clinical trials. Furthermore, a better understanding of the role of each muscle in the occurrence of SR after surgical correction of distinct dentofacial subtypes will enable the development of new techniques to limit post-operative SR in specific subject groups.

| Morphological changes of the perimandibular muscles
The effect of BSSO advancement on the morphology of the pe- This study showed only minor increases in joint reaction force after surgery, making it unlikely that increased joint loading resulting from changes in muscular direction is a causal factor in the occurrence of SR due to progressive condylar resorption. 29

| Functional changes of the perimandibular muscles
Alternations in the functional properties of a neuromusculoskeletal system are often evaluated using EMG and/or kinesiometry. EMG enables the quantification of muscle activation. The magnitudes of the EMG signals change as the neural signalling calls for increased or decreased muscular effort. Although muscle contraction is initiated by neural muscular activation, the resulting movement and generated forces are further determined by contraction dynamics (depending on the kinetics of the joint) and musculoskeletal geometry (determining the moment arms of the different muscles in the system). 38 Wessberg and Epker found that the modified sagittal split tech- Di Palma et al concluded that the improvement in the symmetrical distribution of neuromuscular activity was due to improved stability of the occlusion rather than biomechanical advantages, as the effect was independent of the type of surgical jaw displacement. 26 However, it should be emphasized that these conclusions were drawn from statistically insignificant results and in contradiction with the differences in excitation patterns and neuromuscular adaptation in Angle class II subjects compared to Angle class III subjects as reported by Eckardt et al. 24 Based on the limited available evidence in this review, it can be concluded that functional adaptation in terms of EMG activity occurs in the masticatory muscles after BSSO advancement. However, the net result of these changes on the functional properties of the neuromusculoskeletal system cannot be determined as other variables, as described in the first paragraph of this section, might also be affected by orthognathic surgery. Furthermore, it should be emphasized that the relevance of altered EMG activity in the occurrence of SR remains unclear.

| Limitations of available evidence
The first limitation of the currently available evidence presented in this review is the lack of studies that used 3D analyses to assess skeletal jaw movements and relapse. Studies that met the inclusion criteria relied on the 2D measurements of the lateral cephalograms. As 3D analyses are increasingly becoming the clinical standard and with proven higher accuracy, the use of 3D evaluation of mandibular advancement and SR is strongly recommended for future studies. 2,3,40 Another limitation is the lack of specific subject selection in the majority of studies. Instead of including a general group of class II subjects, it is recommended to include a homogenous subject population that is at risk for SR in order to evaluate potential preventive strategies for SR in future studies.
Lastly, the high heterogeneity of treatment protocols (Table 2) and the reported outcomes regarding morphological and functional changes of the perimandibular muscles (Table 5) did not permit a quantitative meta-analysis.

| CON CLUS ION
In conclusion, the findings of this systematic review have demonstrated that the perimandibular muscles play a role in the occurrence of SR after the advancement of BSSO and may serve as a potential target for the prevention, or at least reduction, of SR in specific subjects. Nevertheless, future research is necessary to (i) develop a better understanding of the role of each muscle group, (ii) develop new therapeutic strategies and (iii) define criteria that allow the selection of subjects that will benefit from these new therapies.

ACK N OWLED G EM ENTS
None.

CO N FLI C T S O F I NTE R E S T
The authors have nothing to declare with regard to conflicts of interest.

E TH I C A L A PPROVA L
Not required.

PATI E NT CO N S E NT
Not required.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.